1. Field of the Invention
The present invention relates to a light emitting apparatus configured to illuminate a display or be used as a flash or the like in a camera, and which uses, in particular, a light emitting diode (LED) as a light source, a method for producing the light emitting apparatus and an assembly incorporating the light emitting apparatus.
2. Description of Related Art
In a light emitting apparatus configured to illuminate a display or be used as a flash or the like in a camera, and which uses, in particular, a light emitting diode (LED) chip as a light source, a potting system or transfer forming has been used in recent years as a process to seal the LED chip with a sealing resin.
With a light emitting apparatus using an LED chip as a light source, because it is usual to mix a colorant, pigment, phosphor or a combination thereof in the sealing resin to adjust color or chromaticity of light emitted from the LED chip, it is important to guarantee provision of a fixed amount of the sealing resin to achieve stable chromaticity.
However, the light emitting apparatus formed by either of the above-mentioned potting system or transfer forming has the following problems.
The first problem is that, although production facilities in the potting system are relatively inexpensive, because the injection time and injection pressure of the sealing resin are controlled by a machine, the amount of the sealing resin or phosphor injected varies according to mechanical accuracy. Also, because the potting system is easily influenced by environmental conditions, it is difficult to control the amount of the sealing resin. Consequently, it is difficult to stabilize chromaticity of the light emitted from the LED chip.
On the other hand, in the transfer forming, it is easier to control the amount of the sealing resin than in the potting system, but there are some problems such as restrictions on utilization of the sealing resin, increment of cost for production facilities because the mold is very expensive, influence of the mold release agent on the optical characteristics of the light emitting apparatus resulting from the fact that such a mold release agent must be mixed in the sealing resin to prevent the sealing resin from becoming adhered to the used mold, low level of adhesion between the sealing resin and other parts, and so on.
Furthermore, it has also been pointed out that, when a light emitting apparatus is formed by mounting an LED chip on a lead frame using transfer forming, there is no way of avoiding increased thickness of the light emitting apparatus thus formed (for reference, see Japanese Patent No. 3137823, paragraph 0007).
The second problem concerns the strength of the sealing resin to protect the LED chip.
Because the sealing resin is configured to be exposed without being covered by a cover or the like so as to output light from the light emitting apparatus, it is easily influenced by external forces. However, when a sealing resin with a high degree of hardness is used in order to reduce the influence of external forces, there is a reliability problem in that parts may become separated and the bonding wires to electrically connect the LED to the electrodes may be subjected to damage and tend to become greatly deformed when the LED chip is sealed by the sealing resin.
The third problem concerns heat-resistance of the sealing resin for sealing the LED chip.
For example, with a conventional sealing resin which might be made of epoxy, because it is difficult for heat emitted from the LED to be released, the light emitting apparatus suffers the undesirable problem of a cycle of “heating up→lowering of VF value→further increment of current→further heating up exceeding absolute maximum specified rate→departure form assured range”.
Some proposals have been made to resolve the above-mentioned problems.
For example, in one proposal a hole is provided in a substrate and an LED chip is mounted on a metallic portion provided on a back surface of the substrate facing the hole to improve heat release characteristics from the LED chip and achieve a thinned light emitting device with improved emission efficiency (for reference, see Japanese Patent No. 3137823).
However, in this proposal, the above-mentioned first problem concerning stabilization of color and second problem concerning strength of the sealing resin have still not been resolved.
There is also a proposal which basically relates the same method as that disclosed in Japanese Patent No. 3137823, more specifically, a method in which a hole is provided in a substrate and a sealing resin of an LED chip may be formed by a transfer-mold (for reference, see Japanese Patent Laid-Open No. 2003-31850).
However, in this proposal, there is no intention at all to resolve the first problem concerning stabilization of color and the second problem concerning strength of the sealing resin.
There is a further proposal to stabilize color by reducing the air bubbles in a sealing resin when forming the sealing resin having low heat resistance and a thinned shape (for reference, see Japanese Patent Laid-Open No. 2000-12576).
However, what is disclosed in this proposal is a structure which presupposes that if there are no air bubbles in a sealing resin, the color is stable. Therefore, if there is the slightest amount of air bubbles, it is difficult to obtain a stable color.